The present invention is in the field of battery technology and, more particularly, in the area of additive compounds for use with high energy electrodes in electrochemical cells.
An electrolyte serves to transport ions and prevent electrical contact between electrodes in a battery. Organic carbonate-based electrolytes are most commonly used in lithium-ion (“Li-ion”) batteries and, more recently, efforts have been made to develop new classes of electrolytes based on sulfones, silanes, and nitriles. Unfortunately, these conventional electrolytes typically cannot be operated at high voltages, since they are unstable above 4.3 V or other high voltages. At high voltages, conventional electrolytes can decompose, for example, by catalytic oxidation in the presence of cathode materials, to produce undesirable products that affect both the performance and safety of a battery. Conventional electrolytes may be degraded by reduction by the electrodes when the cells are charged.
As described in more detail below, solvents, salts, or additives have been incorporated into the electrolyte to decompose to form a protective film called a solid electrolyte interphase (SEI). Depending on the exact chemical system, this film can be composed of organic or inorganic lithium salts, organic molecules, oligomers, or polymers. Often, several components of the electrolyte are involved in the formation of the SEI (e.g. lithium salt, solvent, and additives). As a result, depending on the rate of decomposition of the different components, the SEI can be more or less homogenous.
In past research, organic compounds containing polymerizable functional groups such as alkenes, furan, thiophene, and pyrole had been reported to form an SEI on the cathode of lithium ion batteries. See, e.g., Y.-S. Lee et al., Journal of Power Sources 196 (2011) 6997-7001. These additives likely undergo polymerization during cell charging to form passivation films on the electrodes. The improvement in cell performance using these materials was slight.
Further, certain organic polymers have also been used as bulk electrolyte solvents for lithium ion batteries due to the generally superior chemical stability of polymeric-based solvents as compared to smaller organic molecules, such as organic carbonates. However, practical application of such systems has been limited due to poor ionic conductivity.
For high energy cathode materials, electrolyte stability remains a challenge. Recently, the need for higher performance and high capacity lithium ion secondary batteries used for power sources is dramatically increasing. Lithium transition metal oxides such as LiCoO2 (“LCO”) and LiNi0.33Mn0.33Co0.33O2 (“NMC”) are state-of-the-art high energy cathode materials used in commercial batteries. Yet only about 50% of the theoretical capacity of LCO or NMC cathodes can be used with stable cycle life. To obtain the higher capacity, batteries containing these high energy materials need to be operated at higher voltages, such as voltages up to about 4.7V. However, above about 4.3V, conventional electrolytes degrade and this leads to a significant deterioration of the cycle life. Further, the decomposition of the electrolyte at higher voltages can generate gas (such as CO2, O2, ethylene, H2) and acidic products, both of which can damage a battery.
As disclosed herein, these challenges and others are addressed in high energy lithium ion secondary batteries including cathode active materials that are capable of operation at high voltages (at least about 4.3V) with a carbon-based anode.